Cleaning Gunk from an Aluminum Can Top

A number of years ago, my friend Dan came up with an idea for a device that would clean the tops of soda cans.
He considered getting a patent and marketing the device to potential manufacturers, but he couldn’t come up with a way to create a prototype.
In the meantime, he discovered that a lot of people already have patents for such a tool.
(Don’t get me started on the looseness of patent rules.)

For his 40th birthday, I decided to make him a prototype as a gift.
The device I created isn’t exactly what he envisioned, but it’s similar enough to make the point.

Cleaning an aluminum can top with the prototype device.

The pop top cleaner is about 2.5-inches in diameter and looks like a plastic control knob.
The bottom of the device has a circular ring that mates with the top of a standard aluminum drink can.
Place a piece of tissue paper or a paper towel between the device and the can top.
Push down and rotate to clean the top of the can.

Making a Wax Copy

I needed to determine the dimensions of the top of an aluminum can.
I couldn’t find the specifications on the Internet.
And, unfortunately, the curvy depressions are difficult to measure using calipers.

To make it easier to measure, the lid shape can be inverted by making a wax impression.
Wax is melted in an oven and poured onto a full soda can that has aluminum foil taped around the sides to form a small bowl on the top.

Left: A wax impression of the top of an aluminum can. Right: Using a drawing program to measure the angle of the lip.

Although the above image looks like the top of a can, it’s actually inverted.
That is, the pull tab does not protrude; it’s actually recessed in the wax mold.
It is an empty hole in the shape of a pull tab.
Our mind is so accustomed to a physical pull tab that it interprets the image incorrectly.
It’s an optical illusion.
(See the following picture.)

The wax impression viewed from the side doesn’t present the illusion.

At this point, I thought to myself “Why not just pour plastic resin on a can and machine it a little bit to instantly create the cleaning device prototype?”
Well, I didn’t have any fresh resin.

I took a photograph of the inverted wax copy of the can top and imported it into a drawing application (Microsoft Visio).
In the software, I drew a line to match the angle of the inside of the can lip.
Wait! Darn! The lip is curved!
Oh, well. I'll accept an approximation using a linear angle.

The properties of the line in Visio show the lip angles 78.5 degrees from horizontal.
That’s 90-78.5=11.5 degrees from the vertical.

Machining on a Lathe

The prototype is made from teal-colored Delrin rod scraps obtained from an eBay auction.
Delrin is the brand name for acetal thermoplastic.
It is wear resistant, washable, and FDA compliant.
All three material attributes are important for this device.

The plastic workpiece is placed in a MicroLux lathe fitted with a large (5-inch) chuck, purchased from Little Machine Shop.
The face and sides of the workpiece are cleaned up with standard tools and techniques.

A paper template with a 90-degree corner adjacent to an 11.5-degree angle is made in Visio, printed out, cut to shape, and placed on the carriage.
Although not perfectly precise, this is a simple way to set up the desired angle on the lathe compound to cut the lip.

A paper template with one side lined up to the carriage (vertical arrow pointing down).
The adjacent side of the template (other arrow) can then be used to visually align the compound to the desired angle.
This picture was taken after the cutting process to show the matching angle created on the outside of the workpiece.

After setting the compound angle, the bolts are tightened and the paper template can be removed.

With the lathe spinning, the compound moves the cutting tool towards the workpiece at an angle, shaving away a little bit of the outer diameter of the workpiece.
After each left-to-right pass, the cross slide moves the cutting tool a little more towards the center of the workpiece.
Over time, this repeated process skims off material until the finished outer angle and diameter are created.

The center of the workpiece is drilled to allow a boring bar to cut away the center material.
The edge is a bit more raised up than the center, so that it will press down on the can top, while the center clears the pull tab all the way around.

Boring bar on a lathe removes material from the center of the workpiece.

Finally, the center is drilled through with a 1/4-inch drill.
While this might appear to be an air hole on the finished product, it is actually needed to mount the device to a rotary table for the final steps.

Drilling the center of the workpiece on a lathe.

Machining on the Mill

The workpiece scrap was longer than needed for the finished can-cleaner prototype.
That’s fine. The extra length helps the lathe chuck hold onto the workpiece during cutting.

To remove the excess, I would normally use a cut-off tool on the lathe.
However, the diameter of the workpiece pushes the limits of a miniature lathe.
The cut-off tool needs to be short to be moved past the front of the workpiece, yet it needs to be long enough to cut all the way through the workpiece.

I compromised by using a short cut-off blade to groove the outside of the workpiece.
This provided a nice guide for cutting with a hacksaw.

The hacksaw left a nasty surface which could be faced clean by placing it back onto the lathe.
However, it was just as easy to clean up on a milling machine.

Left: Cutting off excess rod length using a hacksaw. Right: Smoothing the surface on a milling machine.

Although there isn’t much torque involved, a smooth, round, soda-lid cleaner might slip during usage, particularly if the can is wet.
Eight 3/8-inch indentations are made around the circumference by mounting the workpiece to a rotary table and milling holes every 45 degrees (360/8=45).

Cutting grip holes around the workpiece perimeter on a rotary table on a milling machine.

Edges can be surprisingly sharp on newly machined dense plastic.
Although sharp plastic edges are unlikely to cut or cause injury, they may cause discomfort while grabbing and turning the device.

The top edge is rounded by mounting a curved router bit in the milling machine and turning the rotary table.
This isn’t the officially sanctioned usage of a router bit, but it should work okay for light duty on most plastics.

Rounding off sharp edges by using a curved router bit on a milling machine.

Conclusion

The finished prototype mates fairly well with a standard aluminum can.
But, the device isn’t much better at cleaning than just using a paper towel and a finger.

Maybe it could be tweaked with a metal band to really dig into the outer groove?

There are more sophisticated aluminum can lid cleaning designs out there with built-in sponges, wipers, and tab lifting mechanisms.
I wonder if the reason why none are actually produced is because they aren’t very effective?
Or perhaps there is enough variance in can lips to eliminate a generic tool?

Even if it doesn’t work well for everyday use, a handmade object always makes for a nice birthday present.